1. Field of the Invention
The present invention relates to aluminum powder or aluminum alloy powder processing via die compaction and sintering, and in particular, to a method for producing high strength compacts of precipitation-hardened, and possibly dispersion-hardened type aluminum alloys.
2. Prior Art
Aluminum powder compaction and sintering when compared with iron powder processing encounters several difficulties because of the stable oxide film covering the Al powder particles. Most known sintering techniques of aluminum powder alloy compacts involve mixing aluminum powder with other metal powders such as Cu, Mg, Si, that form eutectic liquid phases during sintering. The critical parameter for aluminum powder sintering, i.e., direct metal-to-metal interface contact, has been outlined by Storchheim in U.S. Pat. No. 3,687,657 of Aug. 29, 1972. His technique emphasized that an intimate admixture of the metal powder particles with aluminum particles and a high compacting pressure (40 to 60 tons per square inch) were necessary to establish the metal-to-metal contacts. Later, Kehl and Fischmeister, Powder Metall., vol.23, No. 3, (1980), pg. 113, mentioned the importance of the initial metal-to-metal contacts for completion of the sintering step. These investigators used dendritic copper particles to penetrate the oxide skin of the aluminum. They observed that the dendritic powder reacts within a few degrees above the eutectic temperature.
The oxide layer of the powdered aluminum is quite active with respect to moisture and shows a strong tendency for hydration reactions leading to the formation of both chemically bonded (hydroxides or oxi-hydroxides) and physically adsorbed water.
The prior art has demonstrated that aluminum powder degassing is essential for producing hot worked articles by extruding, forging, rolling and the like. The degassing treatment is normally conducted by exposing canned powder or a green compact to temperatures in the range of 350.degree. to 450.degree. C. in a partial vacuum. The degassing treatment is believed to produce a more thorough removal of contaminants from the powder surface leading to the transformation of the ductile hydroxide into brittle .gamma.-alumina. Subjecting the degassed powder to consolidation will produce a more effective oxide break-up and as a result, an enhanced bonding of the particles is achieved, thereby improving the strength of the material.
According to conventional practice the green compacts are enclosed in welded cans and then degassed. At the completion of the degassing treatment the cans are sealed to retain the vacuum and then subjected to a combination of high temperature and pressure (hot working). Such a process is obviously complicated and, in addition to the fact that it is time consuming and costly, it doesn't offer the possibility of using net shape forming processes.
A previous investigation reported by Lawrence and Forester in Met. Eng. Q., vol. 11, No. 3, (1971), pg. 113, identified the atomizing conditions that allowed the production of low oxide content powders with improved sinterability. Their work also showed that even after seven days of exposure to air at 38.degree. C. with 100% humidity the powders were still sinterable. Accordingly, short time exposure during powder handling should not have an adverse effect on powder surface state after degassing.